feat: support for configurable token lifetime

[ScruffyProdigy]

No description provided.

[lsirac]

Add an integration test where the requested lifetime < 3600s (so no exemption needed).

[clundin25]

__ ERROR collecting system_tests/system_tests_sync/test_external_accounts.py ___
.nox/external_accounts-2-7/lib/python2.7/site-packages/_pytest/python.py:507: in _importtestmodule
    mod = self.fspath.pyimport(ensuresyspath=importmode)
.nox/external_accounts-2-7/lib/python2.7/site-packages/py/_path/local.py:704: in pyimport
    __import__(modname)
E     File "/tmpfs/src/github/google-auth-library-python/system_tests/system_tests_sync/test_external_accounts.py", line 176
E       def test-file_based_external_account_with_configure_token_lifetime
E               ^
E   SyntaxError: invalid syntax

[clundin25]

Sorry I missed this earlier, it seems you have hit a weird edge case. This will create a shared dictionary in all calls, this is not your intent correct?

https://docs.python-guide.org/writing/gotchas/#mutable-default-arguments

[clundin25]

Here is an example in a Python3 shell:

❯ python
Python 3.10.4 (main, Jul 19 2022, 20:41:22) [GCC 11.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> def foo(x, y, d={}):
...     d[x] = y
...     return d
...
>>> foo('x', 'y')
{'x': 'y'}
>>> foo('z', 'g')
{'x': 'y', 'z': 'g'}
>>>

[clundin25]

Agreed to follow up with a fix for this